1. Field of the Invention
The invention relates to a low-pressure feedwater preheater or district heater, which is heated by bleed steam and has two-pass tube bundles of the tube-sheet type of construction, for power station plants and relates in particular to a design for the tube-bundle carrier.
2. Discussion of Background
Low-pressure feedwater preheaters are generally known. They are arranged in a power station plant between the condenser and the deaerator/feedwatertank and serve to preheat the feedwater in a steam/water circuit by heat exchange between steam and feedwater flowing in tubes. To this end, the steam is bled from a low-pressure turbine and directed into the preheater, where it condenses on the tubes and heats the feedwater. In a district-heating power station, the heater serves to reheat heating water which flows through a district-heating circuit.
Air (non-condensable gases) passes with the bled steam as well as from the atmosphere, in particular in the case of preheaters operated in the vacuum range, into the preheater and hinders the heat transfer. This hindrance is removed by deaearation of the apparatus by drawing off these non-condensable gases, for which reason various measures have to be taken in order to carry away the gases as completely as possible.
The feedwater flows via a two-pass system from a water box which is subdivided into two parts and is arranged at one end of the preheater. This two-pass system consists of a bundle of tubes, which are either designed in a straight line together with a reverse flow water box or are of U-shaped design. The water flows from the first part of the water box through the tubes via the revese flow water box or via the U back into the second part of the water box, the cold half of the tubes being designated as cold leg and the hot half being designated as hot leg. In feedwater preheaters of the tube-sheet type of construction, the tube bundles are each firmly anchored in a plate which is arranged in a position adjacent to the water box. Furthermore, the tubes are held and supported by tube carriers, which in each case consist of supporting plates and longitudinal support bars.
The steam bled from the turbine passes through the steam inlet opening into the low-pressure feedwater preheater and is distributed by a baffle plate, so that it first of all flows into an annular steam space between the shell and the tube bundle and then penetrates radially into the tube bundle along the entire length of the latter and condenses on the tube surfaces. There is a free space, the so-called bundle lane, in the center of the tube bundle. Direct inflow of the steam into the bundle lane is prevented by side plates arranged on the outside. Arranged in the bundle lane are one or more deaeration tubes. The latter have a plurality of suction openings through which the non-condensable gases are drawn off. The gases collect in the zone in which the lowest pressure prevails. It has been found that the zone of lowest pressure does not run parallel to the tubes and along the center of the bundle lane but shifts gradually from the center of the bundle to the cold leg of the tube bundle. It therefore runs obliquely from the center of the deflection side of the tubes to the cold inlet side of the tubes at the water box. It is not possible to place a suction tube ideally in this zone of lowest pressure, since such a suction tube would have to be placed at the tube-sheet side on or in the cold leg.
Described in DE 28 20 736 is a feedwater preheater which, in the bundle lane between the cold and the hot leg of the tube bundle, has a partition which extends along the entire length of the tube bundle. Due to the partition, a zone of lowest pressure which does not extend into the tube bundle is obtained in the bundle lane on either side of and in proximity to the partition. Here, suction tubes having deaeration openings through which the non-condensable gases are drawn off are arranged on each side of the partition. This ensures that there are deaeration tubes at the points of lowest pressure in both the hot and the cold bundle half.
The partition, which is continuous over the bundle length, is welded to the side plates of the bundle carriers. The perforated supporting plate consisting of two halves is likewise welded to the side plates and the partition. From the technical point of view, the welded joints are readily suitable for this type of preheater; however, the fabrication of the bundle carriers with this welded construction is very time-consuming, involves a great deal of work and is very costly.